The present invention is directed generally to a counterbalance or motion control valve assembly. The present invention is more particularly directed to a counterbalance valve assembly for use in a hydraulic system of the type which includes a hydraulic actuating cylinder for raising and lowering loads.
Hydraulic systems for raising and lowering loads are well-known in the art. Such systems generally include a hydraulic actuating cylinder which is powered in both directions for raising and lowering a load. Systems of this general nature are usually incorporated into hoists or cranes.
When a load is to be raised, the actuating cylinder is powered through the receipt of hydraulic fluid into a first cylinder port for forcing the cylinder piston, and thus the load, in an upward direction in a conventional manner. For lowering the load, the cylinder is powered in the opposite direction by the receipt of hydraulic fluid through a second cylinder port for driving the cylinder piston, and thus the load, downwardly. Simultaneously, the hydraulic fluid within the cylinder used to raise the load is exhausted through the first cylinder port. Obviously, any load which is being lowered will aid the system hydraulic pump in driving the cylinder piston downwardly and forcing the hydraulic fluid from the first cylinder port. Unfortunately, in certain situations, the load may be sufficiently heavy to cause the cylinder piston to exhaust the hydraulic fluid at a flow rate which exceeds the system pump capacity for filling the cylinder, to thus cause a "runaway" condition to exist.
To prevent a runaway condition from occurring, hydraulic systems of the above-mentioned variety have been provided with a counterbalance valve which is designed to restrict the flow rate of the exhausted fluid. Such counterbalance valves generally include a relief valve which is operative in response to the fluid pressure within the cylinder second port supply line to meter the flow rate of the exhausted hydraulic fluid flowing from the cylinder first port. A decrease in the pressure within the cylinder second port supply line indicates that the load is driving the exhausted cylinder fluid out of the first port at a rate which is greater than the pump supply rate. Hence, to avoid a runaway condition, the flow of hydraulic fluid from the cylinder first port is checked by the relief valve.
While counterbalance valves incorporating relief valves have been generally successful in preventing a runaway condition, they have exhibited some problems. The major shortcoming of prior counterbalance valves has been that the relief valves are extremely sensitive to a decrease in the fluid pressure within the second cylinder port hydraulic fluid supply line so as to cause abrupt checking of the hydraulic fluid being exhausted from the cylinder first port. Such abrupt checking causes the load to drop in a series of abrupt steps rather than in a gradual and continuous manner. Obviously, such a condition is to be avoided to prevent damage to the system as a result of the great degree of inertia which results when a heavy load is abruptly stopped.
It is therefore a general object of the present invention to provide a new and improved counterbalance valve assembly for use in a hydraulic system of the type which raises and lowers a load.
It is a further object of the present invention to provide a new and improved counterbalance valve assembly which provides gradual and continuous exhaustion of hydraulic fluid from a hydraulic cylinder as a load is lowered to thereby provide gradual and continuous lowering of a load.
It is a still further object of the present invention to provide a counterbalance valve assembly which provides gradual variations in the hydraulic cylinder exhausted fluid flow rate responsive to the fluid pressure within the hydraulic cylinder fluid supply line.